1. Field of the Invention
The present invention relates to a planetary speed changing device in which causes of oscillation of input shaft are removed to eliminate any abnormal wear of parts, while suppressing noise and vibration.
2. Description of the Related Art
Hitherto, various speed changing devices incorporating internal-meshing type planetary gear mechanism have been proposed. Among these proposed speed changing devices, a device called cyclo cycloidal speed reducer (registered trademark) is well known. This type of speed changing device has an internally toothed gear with teeth formed by pins or combinations of pin and roller, and an externally toothed gear having trochoidal teeth formed by epitrochoidal parallel curves. The externally toothed gear have inner pins or inner rollers which are loosely fitted therein. The externally toothed gear is rotated by a rotation of an eccentric member fitted in the externally toothed gear so as to revolve along the inner periphery of the internally toothed gear, thereby outputting a torque at a speed which is reduced from the input rotation speed. This type of speed changing device is widely used in various fields, because it can transmit a large torque and because it provides a large speed reducing ratio.
An example of such cyclo cycloidal speed reducer will be explained with reference to FIG. 3. The illustrated speed changing device is arranged such that a reduced rotation speed is obtained at an output shaft 2 when a torque is input through an input shaft 1. This device, however, can be used such that the output shaft 2 is fixed so that a rotation output of a reduced speed is obtained through an internally toothed gear.
A hollow eccentric shaft 3 is fixed to the input shaft 1 by means of a key (not shown) and a key groove 4. The eccentric shaft 3 carries two eccentric members 3.sub.1 and 3.sub.2. Externally toothed gears 5.sub.1 and 5.sub.2 are fitted on the eccentric members 3.sub.1 and 3.sub.2 through rollers 6. Each of the externally toothed gears 5.sub.1 and 5.sub.2 has teeth 7 having trochoidal shape and formed on the outer periphery thereof. An internally toothed gear 8, which serves also as an output casing, is fixed in this case. The internally toothed gear 8 has arcuate teeth provided by outer pins 9 and meshing with the teeth of the externally toothed gears 5.sub.1, 5.sub.2. Each outer pin 9 may carry an outer roller. The externally toothed gears have inner pin-receiving bores 10 which loosely receive inner pins 11 on which are loosely fitted inner rollers. Each of the inner pins is closely fitted in a hole formed in an inner pin holding flange 13. The inner roller 12, however, is not essential and may be omitted. In the illustrated device, the inner pin holding flange 13 is formed integrally with the output shaft 2.
Casings 14.sub.1 and 14.sub.2 are united together with the internally toothed gear 8 clamped therebetween. A pair of input shaft bearings 15.sub.1 and 15.sub.2, which are for supporting the input shaft 1, are provided on both sides of the combination of the externally toothed gears 5.sub.1 and 5.sub.2. The input shaft bearing 15.sub.1 is disposed between the outer periphery of the input shaft 1 and the casing 14.sub.1, while the input shaft bearing 15.sub.2 is provided between the outer peripheral surface of the input shaft 1 and the surface of a recess 13.sub.1 formed in the inner pin holding flange 13. A pair of output bearings 16.sub.1, 16.sub.2 are disposed between the outer peripheral surface of the output shaft 2 and the casing 14.sub.2.
In operation, rotation of the input shaft 1 causes the eccentric members 3.sub.1 and 3.sub.2 to rotate. Since the externally toothed gears 5.sub.1 and 5.sub.2 are prevented from rotating about their own axes due to the presence of inner pins 11 received in the holes 10, these externally toothed gears 5.sub.1 and 5.sub.2 are caused to orbit at a radius e. If the difference between the number of the teeth on each externally toothed gear 5.sub.1, 5.sub.2 and the number of outer pins 9, i.e., the number of teeth on the internally toothed gear 8 is one, the mesh between the outer teeth 7 on the externally toothed gears 5.sub.1, 5.sub.2 and the outer pins 9 as the teeth of the internally toothed gear 8 is offset by one pitch of the teeth. Consequently, the rotation of the input shaft 1 is transmitted to the output shaft through the inner pins 11, with the rotation speed reduced to 1/n (n being the number of teeth of each externally toothed gear 5.sub.1, 5.sub.2) due to the meshing between the externally toothed gears 5.sub.1, 5.sub.2 and the internally toothed gear 8.
This known planetary speed reducing device suffers from the following problems.
A discussion will be made as to the manner in which the input shaft 1 and the output shaft 2 are loaded, with specific reference to FIG. 4. As shown in this Figure, the rotational load Wl applied by the input shaft 1 to the output shaft 2 acts at the end of the input shaft bearing 15.sub.2. On the other hand, the load W2 applied by the externally toothed gears (omitted from FIG. 4) to the output shaft 2 acts on the inner pin 11. At the same time, the load W.sub.3 applied by the externally toothed gears to the input shaft 1 acts on the input shaft 1 as illustrated. Thus, the loads W.sub.1 and W.sub.2 applied to the output shaft 2 act on points which are on the same side of the output shaft bearings 16.sub.1, 16.sub.2 as the input shaft 1, so that the output shaft 2 receive these loads in a cantilevered manner. Consequently, a moment is produced to cause the axis of the output shaft 2 to be inclined at an angle .beta. to the axis O.sub.1 of the shaft under no load.
On the other hand, the load W.sub.3 applied to the input shaft 1 acts to produce a moment which, in combination with the inclination of the output shaft 2, causes the input shaft 1 to be inclined at an angle .beta. to the axis O.sub.1.
Consequently, both the input and output shafts rotate with their axes deviated from the axis of rotation, resulting in problems such as extraordinary wear of parts and generation of noise and vibration.
Referring now to FIG. 5, when a radial load F is applied from the exterior to the input shaft 1, the input shaft 1 is inclined at an angle .beta.' with respect to the original axis O.sub.1, while the output shaft 2 also is inclined at an angle .alpha.' with respect to the original axis O.sub.1, thus causing abnormal wear and generation of vibration and noise.
The inclinations of the output shaft 2 and the input shaft 1 are attributable to the fact that the input shaft 1 receives the load W.sub.3 from the externally toothed gears 5.sub.1, 5.sub.2 and that the load W.sub.3 is supported by the input bearings 15.sub.1 and 15.sub.2.
In the known planetary speed changing device described above, the balance of load is attained when torque is equally distributed to both externally toothed gears 5.sub.1 and 5.sub.2. However, since these two externally toothed gears are not disposed on the same plane, a moment is produced to act on the eccentric members 3 or the input shaft 1 by the loads acting on the externally toothed gears 5.sub.1, 5.sub.2. It has therefore been necessary to provide input shaft bearings 15.sub.1, 15.sub.2 for supporting the input shaft 1. The moment acting on the input shaft 1 is the product of the force acting on the externally toothed gears 5.sub.1, 5.sub.2 and the distance between the externally toothed gears 5.sub.1 and 5.sub.2. The externally toothed gears 5.sub.1 and 5.sub.2 are supported by the eccentric members 3 and the rollers 6. These eccentric members 3 and the rollers 6 are required to have certain minimum lengths from the view point of load capacity. Thus, there is a practical limit in the reduction of the space between two externally toothed gears 5.sub.1 and 5.sub.2.